Honestly, the Answer Hub really isn’t the place for this sort of question. Nobody here is going to tell you how to make a game.
You’d be better off asking a more specific question that concerned some aspect of the development that you’re struggling with.
I don’t mean to be overly critical, but everyone here hast spent hundreds of hours to develop just a rudimentary appreciation of the craft, so to condense that into a single answer isn’t really possible.
However, I will briefly do what I can to answer your question.
It appears to me that you’re going to need some kind of variable (likely a float) to track your light level. This will effectively be a health bar, I presume, so this will be intrinsically linked to your end state. That is to say that when the light is extinguished - it’s game over.
You would perhaps achieve this with a simple setter function that triggered an endgame when the currentLightLevel < 0 .
The more complicated aspect of your requirements are likely going to require collision detection.
Look into colliers - specifically Collider.OnTriggerEnter or Collider.OnColissionEnter. There aught to be ample sources of information on this around, as it is a foundation of many aspects of game development.
Effectively, what you could do is add a collider on your player that detects colission with an enemy, at which point your health is reduced by a set amount. This is a really simplistic approach, but it works.
Here’s a video tutorial on implementing collision events: 3D Physics - Unity Learn
As for enemies - if you’re not confident enough to program an AI using the behavior tree, then I would instead suggest some kind of paroling enemy that moves from point to point. You could use a trigger to detect the player, and then perhaps use Vector3.MoveTowards to move the enemy towards the player.
I’m afraid I can’t really answer with any more detail than this, as I know not your requirements, your current progress or your skill level, but, if you truly wish to complete this project, then I implore you to spend a few more hours in the engine just messing around and tinkering with things to see what might work for you.
Also, don’t ever expect perfection on your first try, or any subsequent attempt for that matter, as we always set goals way above our abilities.